But air pollution doesn't respect international borders. And a pair of recent studies has demonstrated this in striking fashion, showing that the pollution is likely increasing the strength of cyclones forming over the Pacific Ocean and even spreading detectable levels of contaminants all the way to the Western U.S.

The research on air pollution and Pacific cyclone activity, published today in Nature Communications, involved the use of statistical modeling to calculate the effect of air pollution on overall storm formation trends over the Pacific. The research team, led by Yuan Wang of Texas A&M, created a model to simulate the role of temperature, precipitation, cloud cover and other factors in affecting the strength and frequency of Pacific cyclones.

Then they ran the model under two scenarios: One in which the air over Asia was clean, and one that represented the enormous increases in air pollution in recent years, present in most East Asian countries but led by China, which has some of the highest levels of particulate airbone pollution worldwide.

The dynamics of cloud formation are quite complex, but the study's conclusions were surprisingly straightforward. This air pollution, the model showed, has powerful consequences for cyclone formation, increasing overall precipitation over the Northwest Pacific by 7 percent over what it would be otherwise. The particulate matter, the researchers found, is also producing a regional greenhouse gas effect, siginificantly contributing to climate change.

These findings are, naturally, pretty worrisome. But for U.S. readers, the results of the study on the global spread of Chinese air pollution, published yesterday in the Proceedings of the National Academy of Sciences, might be even more troubling, both because they show the effects of Chinese pollution here in the U.S. and because they trace much of the responsibility back to American consumers, who buy a large proportion of the goods manufactured in China.

The research team, made up of both American and Chinese scientists, also used simulations to determine how much of the particulate matter emitted in China rides a seris of prevailing winds known as Westerlies across the Pacific and enters U.S. airspace. They found that, especially in Western U.S. states, significant amounts of the air pollution present could be traced to China. This was true even though the data they studied came from 2006—since then, air pollution in China has climbed rapidly.

On the days with the strongest Westerlies—which occur most often during the spring—between 12 and 24 percent of the sulfate-based air pollution over the Western U.S. was originally generated in China. That was also true for four to six percent of carbon monoxide and two to five percent of ground-level ozone. As a result, they estimated, in 2006 the Los Angeles area experienced one extra day of ozone levels that exceeded EPA standards for air quality.

But this isn't the whole picture. A significant portion of the goods made in China end up in the U.S. As the researchers note, we've outsourced our manufacturing to China, for a variety of reasons. Many of these involve the global labor market and the costs of production, but others are related to China's overall weaker environmental regulations that allow companies to produce goods more cheaply.

Because of this, it's also important to look at the air pollution over China that was generated to make products for customers in the U.S. and other countries. The researchers did this by calculating how much of Chinese manufacturing fits within this category and how much pollution faced by the Chinese people is a direct result of consumer demands from other countries.

The results aren't pretty. An estimated 36 percent of manmade sulfur dioxide, 27 percent of nitrogen oxide, 22 percent of carbon monoxide and 17 percent of black carbon over China are the result of manufacturing goods for export. About a fifth of each of these was associated with products exported to the U.S. in particular. These pollutants have a range of acute effects on human health and can drive the formation of acid rain.

The lesson of all this? Perhaps that China is responsible for some of the pollution over the U.S., but also the reverse: The American demand for cheap products fuels Chinese pollution in the first place.

"We've outsourced our manufacturing and much of our pollution, but some of it is blowing back across the Pacific to haunt us," Steve Davis of UC Irvine, a co-author, said in a press statement. "Given the complaints about how Chinese pollution is corrupting other countries' air, this paper shows that there may be plenty of blame to go around."

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